For different kinds of electric rotary motors, various measures have been taken to improve motor efficiency. With regard to rotors, improvement by transpositions of materials of electromagnetic steel plates used as core parts or conductor bars and improvement in the space factor of conductor bars or modifications of shapes of cores are being considered.
A prior art example of a rotor for an electric motor is shown in FIGS. 14A, 14B and 15. A core 1 includes a plurality of thin electromagnetic steel plates. As shown in FIG. 15, each of the core plates has a core shaft hole 12 at the center and a plurality of approximately trapezoidal core slots 11 arranged circumferentially. The plates are laminated to each other to form the core 1.
The rotor also includes end rings 2 whose shapes are approximately the same as the plates of the core 1. The end rings 2 are attached at both sides of the core 1 in the shaft axis direction. The rotor shaft 3 extends through the core shaft holes 12 of the core plates and through the corresponding holes of the two end rings 2. The core slots 11 of the core 1 and the end rings 2 accommodate the conductor bars 4, so that the conductor bars 4 extend through core slots 11 of both the core 1 and end rings 2. In this arrangement, a squirrel-cage rotor for an electric motor is manufactured by joining the end rings 2 and conductor bars 4 by brazing or other various kinds of methods to provide electrical continuity throughout.
For this rotor structure, the end rings 2 and the conductor bars 4 are prepared as separate parts; and they are bonded together after the core 1 is assembled. Alternatively, a die-casting process may be used as another option. In the die-casting process, the end rings 2 and the conductor bars 4 are made directly by forcing molten metal into core slots 11 which are regarded as cavities for casting purposes.
Rotary imbalance of the squirrel-cage rotor produced by the above processes is one factor in vibration and noise in an electric motor that uses such a rotor. Hence, modifications of the rotor are conducted in order to reduce such imbalance and thereby improve balance and reduce motor vibration. These modifications for balance are becoming essential due to recent requirements for higher efficiency.
There are two different methods for modifications of the rotor for improving balance. One balancing approach is to cut part of the rotor by drilling or shaving to remove weight, and the other approach is to attach one or more balance weights. The drilling or shaving approach may be chosen unless wastes by cutting causes problems, and the use of balance weights may be chosen if the waste matter is disadvantageous.
Explanation is given of an example of the method of attaching balance weights hereinbelow. In order to reduce the number of parts and the number of steps for attaching the parts together, a plurality of balance weights are manufactured in the form of ring-shaped plates at the center of each of which is a hole. These plates, however, have respectively different thicknesses and therefore different mass and weight. A projection is formed at a surface of the edge of the rotor so that the projection can be fitted into the central hole of any of the balance weights.
After measuring quantity of imbalance of a rotor without balance weights, one or a plurality of the ring-shaped balance weights are fitted onto the projection so as to improve balance within the prescribed range to offset the quantity of imbalance. In a case where a plurality of balance weights are attached, a plurality of balance weights whose thickness are the same or are respectively different are available. To prevent detachment, the ring-shaped balance weights are kept attached to the projection of the rotor, for example, by caulking. Alternatively, apart of the projection from the surface of the rotor is deformed to be larger than the central hole of the attached weight (s).
Regarding this problem of rotor modification to improve balance, reducing the number of manufactured parts and simplifying assembly procedures are desirable. For example, a method of applying putty to an outer surface of the coils and hardening the putty has been proposed. However, this balancing technique has a potential for the putty to detach due to centrifugal force. Putty detachment has occurred especially when the attaching force of putty has been weakened by deterioration of the putty over time.
In view of the above mentioned detachment problem, JP-A-H07-143709A proposes that a cylinder is attached to a component of rotor and putty intended to improve balance is applied and hardened on an inner surface of the cylinder. This structure has solved the detaching putty problem because the putty at the inner surface is being forced against the inner surface by centrifugal force. Also, JP-A-2007-336737A proposes that a sleeve projected in the axial direction of the rotor can be cut by drills or other means to change weight distribution so as to improve balance.